Circadian Population Pharmacokinetics of 5-Fluorouracil (5-FU) in Patients with Metastatic Colorectal Cancer

To estimate the population pharmacokinetic parameters of 5-FUin patients with advanced colorectal cancer using circadian changekinetics .

Patients and Methods:

Eighty five patients (32 females, 53 males) were enrolled onto this study. All patients received folinic acid (200 mg/m³) by intravenous infusion over 2 h followed by a 5-FU loading dose (400 mg/m³) then continuous infusion (600 mg/m³) for 22 h. This whole regimen was repeated on day 2 and was given on a 14 day cycle. Plasma 5-FU determinations were performed by high-performance liquid chromatographic with ultraviolet absorbance detection. Pharmacokinetic analyses were performed using the NONMEM computer program trough the Visual-NM graphical interface. An open one-compartment pharmacokinetic model with zero order input rate was used to describe the kinetics of 5-FU; moreover, circadian time-dependent changes in 5-FU concentrations were taken into account in the model

Results:

The circadian model for 5-FU clearance changes was defined as the sum of two cyclic components; the amplitude of the first cyclic component (over 24 h) was about 30% of the average clearance and the amplitude of the second cyclic component (over 12 h) was about 50% of the amplitude of the first component. The acrophase (peak) times of the first and the second periodic component were 4.2 and 0.41 h AM, respectively. The potential sources of variability on the population parameters (65 patients) was investigated using patient’s sex, body area, age, body weight, height, and serum creatinine as covariables.

Only the estimated clearance circadian changes were different for the two sexes. The population parameter estimates of mean clearance (CLmean) and initial volume of distribution (V), were as follows: the male subgroup showed a CLmean value twice larger (125 L/h) than the value observed in the female subgroup (65 L/h), and V = 21 L. A validation group of 20 additional patients was used to evaluate the predictive performances of the population parameters. The individual pharmacokinetic parameters were computed by means of a Bayesian fitting procedure. From the resulting individualized parameter values, concentrations of 5-FU in the plasma were calculated. To evaluate the performance of the Bayesian estimation, the experimental concentrations were compared to the predicted ones.

A chronomudulated schedule delivery of 5-FU should be performed, using a perfusion rate inversely proportional to the circadian variations of clearance in order to maintain stable 5-FU plasma levels. A such treatment schedule may result in increased effectiveness of the treatment and a decreased occurrence of drug-associated side-effects. The present study develops a complete procedure to efficiently estimate 5-FU clearance in order to optimize dosage regimen in an individual patient.